Immersion nozzle

ABSTRACT

The invention presents an immersion nozzle having corrosion resistance to steel of high oxygen content or type of steel treated with calcium, and capable of preventing adheres and deposits of molten steel and oxide inclusions on the nozzle inner wall, in which at least a part of its portion contacting with molten steel is composed of a refractory material containing 50 wt. % or more of a principal component including one or more types of magnesia, alumina, spinel, zirconia, mullite, and silica, and also 0.3 to 15 wt. % of boron nitride, and an organic binder.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an immersion nozzle, and moreparticularly to an immersion nozzle for pouring molten steel in atundish into a mold in continuous casting, or an immersion nozzle madeof a refractory material having high corrosion resistance, and capableof suppressing adheres and deposits of matrix metal or nonmetallicinclusion on the nozzle inner wall when used for a long time.

[0003] 2. Description of the Related Art

[0004] In continuous casting, immersion nozzles are used as long nozzleand air seal pipe used between the ladle and tundish, or nozzle forpouring molten steel in the tundish into the mold. These immersionnozzles are required to have such characteristics as spallingresistance, wear resistance and corrosion resistance to molten steel orslag depending on the environments of use.

[0005] To meet such requirements, hitherto, alumina-graphite material oralumina-silica-graphite material has been used as the refractorymaterial for immersion nozzle.

[0006] In such alumina-silica-graphite refractory material, however, incasting of steel of high oxygen content or type of steel treated withcalcium, alumina and silica are eroded by FeO and CaO, and cannotwithstand use for a long time.

[0007] Accordingly, reducing the content of silica which is relativelylow in corrosion resistance, an alumina-graphite refractory material isused, but the resistance of graphite to oxidation and corrosion is low,and sufficient durability is not obtained.

[0008] Besides, in the alumina-graphite or alumina-silica-graphiterefractory material, the inclusion due to Al oxidation or slag in moltensteel adheres and deposits on the inner wall of the immersion nozzle,and the nozzle is likely to be clogged.

[0009] If the immersion nozzle is clogged, it is impossible to controlthe flow rate of molten steel, and operation of continuous casting isdifficult, and moreover as the adheres and deposits on the inner wall ofimmersion nozzle are separated during operation, the quality of productsteel is lowered.

[0010] As means for preventing such clogging of immersion nozzle, amethod of blowing inert gas into the immersion nozzle is proposed.According to this method, adheres and deposits of oxide inclusions suchas alumina cluster on the nozzle inner wall can be prevented, but finebubbles from the blowing gas are taken into the molten steel, andpinholes may be formed, or defects of steel may occur due to entrappingof inclusions. This problem is caused by fine bubbles taken into themolten steel, and it is hence very difficult to solve.

[0011] Among other means, from the viewpoint of material of immersionnozzle, as disclosed in Japanese Patent Application Laid-Open No.56-139260, an immersion nozzle composed of a refractory materialcontaining 5 to 80% of boron nitride has been proposed. More recently,as disclosed in JP-A No. 10-314905, immersion nozzles made of refractorymaterial not containing carbon or lowered in carbon content are proposedin order to prevent adhesion of alumina.

[0012] The present inventors noticed the material of the immersionnozzle as possible means for solving the problem of nozzle clogging, andattempted to improve the existing material.

[0013] That is, in the immersion nozzle composed of a refractorymaterial containing 5 to 80% of boron nitride proposed in JP-A No.56-139260, since it is based on the use of graphite, it involved thetechnical problem of insufficient effect on corrosion resistance andprevention of clogging.

[0014] Or in the immersion nozzle made of refractory material free fromcarbon or lowered in carbon content, although there is an effect ofprevention of adhesion sticking of alumina and other oxide inclusion onthe nozzle inner wall, there is still a technical problem ofinsufficient prevention of adhesion of molten steel itself.

[0015] Thus, in the conventional technologies about the materialproposed as means for preventing nozzle clogging, various problemsexisted, and the effect for preventing clogging was not sufficient.

[0016] The invention is devised to solve the problems of the prior arts,and it is hence an object thereof to present an immersion nozzle havingcorrosion resistance to steel of high oxygen content or type of steeltreated with calcium, and capable of preventing adheres and deposits ofmolten steel and oxide inclusions on the nozzle inner wall.

SUMMARY OF THE INVENTION

[0017] In the immersion nozzle of the invention, at least a part of itsportion contacting with molten steel is composed of a refractorymaterial containing 50 wt. % or more of a principal component includingone or more types of magnesia, alumina, spinel, zirconia, mullite, andsilica, and also 0.3 to 15 wt. % of boron nitride, and an organicbinder.

[0018] This refractory material is low in coefficient of expansion, andsmall in wettability to molten steel, and moreover since boron nitrideexcellent in wear resistance and lubricity is used, clogging of nozzleis prevented, and an immersion nozzle excellent in durability ispresented.

[0019] In the immersion nozzle of the invention, further, a thin walllayer of 2 to 15 mm in thickness is formed by this refractory material.

[0020] The thin wall layer is preferred to be formed in the specifiedthickness, at least in the portion contacting with the molten steel,from the viewpoint of nozzle clogging preventive effect and structuralpeeling of refractory material.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a sectional view showing an example of immersion nozzlefor continuous casting according to the invention;

[0022]FIG. 2 is a sectional view showing an example of immersion nozzlefor continuous casting according to the invention.

DESCRIPTION OF THE PREFERRED EXAMPLES

[0023] The invention is more specifically described below.

[0024] The refractory material used in the immersion nozzle of theinvention contains 50 wt. % or more of a principal component includingone or more types of magnesia, alumina, spinel, zirconia, mullite, andsilica, and also 0.3 to 15 wt. % of boron nitride, and an organicbinder.

[0025] Thus, by containing boron nitride in the refractory material,cracks and other breakages hardly take place, and the immersion nozzlecan prevent adheres and deposits of molten steel and oxide inclusion onthe nozzle inner wall. The boron nitride is low in coefficient ofexpansion, and is small in wettability to molten steel, and moreover itis excellent in wear resistance and lubricity.

[0026] If the content of boron nitride in the refractory material isless than 0.3 wt. % , sufficient lubricity of refractory material is notobtained. Or if the content of boron nitride in the refractory materialexceeds 15 wt. %, the sinterability of the refractory material drops andthe strength declines, while the cost is increased.

[0027] In the refractory material, one or more types of magnesia,alumina, spinel, zirconia, mullite, and silica is contained as principalcomponent.

[0028] Herein, magnesia, spinel, zirconia, and mullite are high incoefficient of thermal expansion, and the refractory material may becrack or breakages during use of the nozzle. However, these oxidematerials are high in melting point and high in hardness, and thecorrosion resistance to FeO and CaO is excellent as compared with thatof alumina or silica.

[0029] In these oxide materials, therefore, by blending a specifiedamount of boron nitride and organic binder, the coefficient of thermalexpansion can be lowered, so that the nozzle hardly broken and excellentin corrosion resistance is obtained.

[0030] The principal component composed of one or more types ofmagnesia, alumina, spinel, zirconia, mullite, and silica is preferred tobe contained by 50 wt. % or more in total.

[0031] If the total content of one or more types of magnesia, alumina,spinel, zirconia, mullite, and silica is less than 50 wt. % , sufficientcorrosion resistance of refractory material is not obtained. Further, toprevent digestion of magnesia, it is preferred to contain silica, andthe content of silica is preferably smaller than that of magnesia.

[0032] The refractory material used in the immersion nozzle of theinvention does not contain graphite unlike the conventional nozzle madeof alumina-graphite material or the like. Accordingly, oxidizing gas isnot generated by the reaction between the graphite and other ingredientsin the refractory material during use, and the aluminum in the moltensteel is not oxidized, and formation of alumina can be prevented. Hence,product steel of high quality is obtained.

[0033] An organic binder is also contained in the refractory material.The organic binder is intended to lower the coefficient of expansion ofthe refractory material, and prevent crack or breakage during use of thenozzle. The organic binder is not particularly limited, and phenol resinand other ordinary binder may be used.

[0034] The refractory material is preferred to be formed in a thin walllayer of 2 to 15 mm in thickness in the portion contacting with themolten steel of the immersion nozzle.

[0035] If the thickness of the thin wall layer is less than 2 mm, thecorrosion resistance is not sufficient, and the nozzle cloggingpreventive effect is not enough. On the other hand, if the thickness ofthe thin wall layer exceeds 15 mm, the refractory material may causestructural peeling.

[0036] The thin wall layer is not required to be formed on all of theportion contacting with the molten steel, but may be formed at least inthe nozzle inner wall in order to prevent clogging of nozzle.

[0037] The structure of the immersion nozzle of the invention is notparticularly limited, and examples of structure of refractory in theimmersion nozzle may include structures shown in FIG. 1 and FIG. 2.

[0038] That is, as shown in FIG. 1, a nozzle main body 1 of theimmersion nozzle is composed of an alumina-graphite refractory materialgenerally used in the immersion nozzle, and a powder line portion 2 iscomposed of zirconia-graphite refractory material. In a nozzle innerwall 3, a nozzle outer wall 4, and a nozzle inner hole wall 5 of theimmersion nozzle, a thin wall layer of refractory material containingboron nitride of the invention is formed.

[0039] In FIG. 2, a thin wall layer of refractory material containingboron nitride of the invention is formed only in the nozzle inner wall3.

[0040] The manufacturing method of the immersion nozzle of the inventionis not particularly limited, but the following example may be presented.

[0041] First, materials of the refractory material are kneaded in amixer. The obtained mixture is charged into a molding pattern, and ismolded by CIP molding, die press or the like. The obtained molded pieceis dried, and burned in non-oxidizing atmosphere. After burning, asrequired, it is processed into final shape, and the intended immersionnozzle is obtained.

[0042] The refractory material used in the immersion nozzle of theinvention contains boron nitride, and hence the mixture of the materialsis excellent in lubricity, and it is easier to work in the moldingprocess as compared with the refractory materials free from carbon.

[0043] It is therefore possible to form integrally in the inner wall ofthe nozzle main body made of alumina-graphite refractory material. As aresult, structurally, the spalling resistance is enhanced.

EXAMPLES

[0044] The invention is further explained by referring to preferredexamples, but it must be noted that the invention is not limited to theexamples alone.

Example 1

[0045] In a nozzle main body composed of 36 wt. % of alumina, 30 wt. %of graphite, 25 wt. % of organic binder, and 9 wt. % of silicon carbide,a refractory material in the blending composition in samples No. 1 to 4of the invention shown in Table 1 was formed in the inner wall of theimmersion nozzle as 5 mm thick thin wall layer, and an immersion nozzleas shown in FIG. 2 was manufactured.

[0046] Using this immersion nozzle, casting was tested in an actualfurnace. A type of steel containing 0.03 wt. % of Al was used. As thecontinuous casting machine, a two-strand type machine was used, andinstalling the immersion nozzle in No. 1 strand, 5ch (casting time about170 minutes) casting test was conducted 5 times.

[0047] After the test, in the straight shell portion of the nozzle, theadhere thickness was measured at three sections, and the average wasdetermined. The average of five tests is shown as deposit thickness inTable 1.

Comparative Example 1

[0048] An immersion nozzle in the blending composition of comparativesample No. 1 shown in Table 1, that is, 36 wt. % of alumina, 30 wt. % ofgraphite, 25 wt. % of organic binder, and 9 wt. % of silicon carbide,was manufactured without forming thin wall layer, and it was installedin No. 2 strand of the continuous casting machine, and casting wastested.

[0049] The deposit thickness measured same as in example 1 is shown inTable 1. TABLE 1 Blending composition (wt. %) Deposit Boron OrganicSilicon thickness nitride Magnesia Alumina Spinel Zirconia Silica binderGraphite carbide (mm) Sample No. 1  1  4 84 — — 1 10 — — 1.0 of 2  5 — —85 — — 10 — — 0.8 example 1 3 10 — — — 80 — 10 — — 0.5 4 10 39 — 40 — 110 — — 0.9 Sample No. 1 — — 36 — — — 25 30 9 13.7 of comparative example1

[0050] As shown in Table 1, the deposit thickness of sample No. 1 to 4of the invention was 1.0 mm or less, being less than {fraction (1/10)}of sample No. 1 of the comparative example, and the deposits and adheresdecreasing effect was noted.

Example 2

[0051] In a nozzle main body composed of 36 wt. % of alumina, 30 wt. %of graphite, 25 wt. % of organic binder, and 9 wt. % of silicon carbide,a refractory material in the blending composition in samples No. 1 to 5of the invention shown in Table 2 was formed in the inner wall of theimmersion nozzle as 5 mm thick thin wall layer, and an immersion nozzleas shown in FIG. 2 was manufactured.

[0052] Using this immersion nozzle, casting was tested in an actualfurnace. A type of high oxygen steel (oxygen concentration: 200 ppm) wasused. As the continuous casting machine, a two-strand type machine wasused, and installing the immersion nozzle in No. 1 strand, 1ch (castingtime about 60 minutes) casting test was conducted 3 times.

[0053] After the test, the maximum melt loss thickness of the inner wallwas measured in the section of the straight shell portion of the nozzle.Results are shown in Table 2.

Comparative Example 2

[0054] An immersion nozzle in the blending composition of comparativesamples No. 1 to 5 shown in Table 2 was manufactured without formingthin wall layer.

[0055] This immersion nozzle was installed in No. 2 strand of thecontinuous casting machine, and casting was tested in the same conditionas in example 2.

[0056] After the test, the maximum melt loss thickness of the inner wallwas measured in the section of the straight shell portion of the nozzle.Results are shown in Table 2. TABLE 2 Blending composition (wt. %) BoronOrganic Silicon Max melt loss nitride Magnesia Alumina Spinel ZirconiaSilica binder Graphite carbide thickness (mm) Sample No. 1  1 4 84 — — 110 — — 0.5 of 2  5 — — 85 — — 10 — — 0.2 example 2 3 10 — — — 80 — 10 —— 0.1 4 10 39  — 40 — 1 10 — — 0.2 5 10 4 75 — — 1 10 — — 0.6 Sample No.1 — — 36 — — — 25 30 9 23 of 2 10 4 45 — — 1 10 30 — 22 comparative 3 104 65 — — 1 10 10 — 20 example2 4 10 4 70 — — 1 10  5 — 13 5 10 4 74 — —1 10  1 — 7

[0057] As shown in Table 2, in continuous casting of high oxygen steel,the maximum melt loss thickness of the inner wall of the immersionnozzle was 0.6 mm or less in samples No. 1 to 5 of the example, beingless than {fraction (1/10)} of samples No. 1 to 5 of the comparativeexample, and the corrosion resistance to high oxygen steel was improved.

[0058] In particular, in comparative example samples No. 1 to 4containing graphite by 5 wt. % or more, the maximum melt loss thicknesswas more than 20 times that of the example samples, and it was confirmedthat the corrosion resistance is lower in the refractory containinggraphite.

[0059] Therefore, samples of the example not containing graphite isrecognized to be higher in corrosion resistance to high oxygen steel ascompared with the comparative samples of the conventional parts.

Example 3

[0060] In a nozzle main body composed of 36 wt. % of alumina, 30 wt. %of graphite, 25 wt. % of organic binder, and 9 wt. % of silicon carbide,a refractory material in the blending composition in samples No. 1 to 5of the invention shown in Table 2 was formed in the inner wall of theimmersion nozzle as 5 mm thick thin wall layer, and an immersion nozzleas shown in FIG. 2 was manufactured.

[0061] Using this immersion nozzle, casting was tested in the samecondition as in example 2.

[0062] After the test, the nozzle porosity was measured in accordancewith JIS R 2205 (Measuring method for apparent porosity, waterabsorption specific gravity of refractory bricks). Moreover, the bendingstrength was measured in accordance with JIS R 2213 (Testing method formodulus of rupture of refractory bricks). Results of porosity andbending strength are shown in Table 3.

Comparative Example 3

[0063] In a nozzle main body composed of 36 wt. % of alumina, 30 wt. %of graphite, 25 wt. % of organic binder, and 9 wt. % of silicon carbide,a refractory material in the blending composition in comparative samplesNo. 6 and 7 shown in Table 3 was formed in the inner wall of theimmersion nozzle as 5 mm thick thin wall layer, and an immersion nozzleas shown in FIG. 2 was manufactured.

[0064] In this immersion nozzle, the porosity and bending strength weremeasured same as in example 3. Results of porosity and bending strengthare shown in Table 3. TABLE 3 Blending composition (wt. %) Boron OrganicSilicon Porosity Bending strength nitride Magnesia Alumina SpinelZirconia Silica binder Graphite carbide (%) (MPa) Sample No. 5 10 4 75 —— 1 10 — — 18.7 15 of example 3 Sample No. 6 20 4 65 — — 1 10 — — 24.62.6 of 7 30 4 55 — — 1 10 — — 36.7 1.4 comparative example 3

[0065] As shown in Table 3, the porosity of sample No. 5 of theinvention was 18.7%, but it was over 20% in comparative samples No. 6and 7 containing boron nitride by 20 wt. % or more.

[0066] As a result, the nozzle bending strength was lower in comparativesamples No. 6 and 7, being less than 1/5 of sample No. 5 of the example.

[0067] Therefore, as the content of boron nitride increases, theporosity increases and the nozzle bending strength is lowered, and whenexceeding 15 wt. %, it is recognized that sufficient strength for nozzleis not obtained.

[0068] As described herein, by using the immersion nozzle of theinvention, the corrosion resistance is enhanced against the steel ofhigh oxygen content or type of steel treated with calcium.

[0069] Moreover, adheres and deposits of molten steel and oxideinclusions on the nozzle inner wall can be prevented, and clogging ofnozzle is prevented, so that the nozzle can be used continuously for along period.

[0070] Therefore, by using the immersion nozzle of the invention, laborand time required in maintenance of nozzle in the casting process can besaved, and steel of high quality can be manufactured also in high oxygensteel or the like.

What is claimed is:
 1. An immersion nozzle, wherein at least a part ofits portion contacting with molten steel is composed of a refractorymaterial containing 50 wt. % or more of a principal component includingone or more types of magnesia, alumina, spinel, zirconia, mullite, andsilica, and also 0.3 to 15 wt. % of boron nitride, and an organicbinder.
 2. The immersion nozzle according to claim 1 , wherein a thinwall layer of 2 to 15 mm in thickness is formed by said refractorymaterial.